Category Archives: Journalism

How oil is formed

Posted on by

Oil, natural gas and coal are known as fossil fuels. They are extremely useful substances and humans spend lots of time and energy to extract them from the earth. But why are they called fossil fuels, when and how did they form?

In an town or village in Ireland on a cool evening there is often a delicious pungent smell filling the air. It comes from the smoke from peat burning on open fires in houses and pubs. Peat is thousands of years old and is compressed plant matter formed in wet areas called bogs. As plants grow and die in a swampy area, their remains build up over the years. Underwater there is little oxygen, so the plant material doesn’t get eaten by bacteria of fungi like they would in the soil. Eventually thick layers build up and can be dug and dried to use as fuel.

In Ireland the layers of peat are on the surface and so easy to get at. What would happen if they got buried under layers of sand and mud over millions of years? As it got buried deeper, the pressure of the rock above would squeeze the peat flat, pushing out much of the water. The rock will warm up as it gets buried deeper (the earth is hot inside) and this will cause chemical reactions to change the peat into coal. Coal is fossilised plant material. It may also contain fossils that record shape of the plants that used to live there.

Plants extract carbon from the atmosphere and combine it with water and other elements to make leaves, stems roots and so on. Plant material is therefore a mixture of the elements Carbon, Hydrogen and Oxygen, with much smaller amounts of Nitrogen, Sulphur and traces of others. As the plant matter is buried and heated, the hydrogen and oxygen is driven off, leaving behind the carbon. Coal comes in different grades, depending on how much is was heated. The lowest grade, known as brown coal or lignite contains only 60-75% Carbon. The highest grade, anthracite has over 90% carbon and so contains much more energy per kilogram. It is a valuable fuel.

What about oil? This forms in a similar way – organic matter accumulates in a place with little Oxygen, is buried, ‘cooked’ and turned into a useful substance and store of energy. Coal forms from peat which is plant matter in wet places on land. Oil forms from organic matter that settled on the sea bed to form a substance called oil shale.

There’s a wide source of organic matter that sinks to the bottom of the sea. Plankton is the name for an enormous range of life that floats in the sea, ultimately powered by photosynthesis. It’s an amazing source of energy and powers many of the food webs within the oceans. In the summer, huge areas of ocean turn green as plankton grown in the sun. This green phytoplankton uses photosynthesis to grow. Zooplankton are tiny floating animals that eat the phytoplankton. Small pink shrimp called krill eat the plankton and are in turn eaten by blue whales, the largest living creatures.

Plankton that doesn’t end up in whales or other animals eventually sinks to the sea bed. Plant material from the land also flows into the sea, including spores and pollen, also resins and lipids, parts of plants that decompose slowly.

Imagine yourself in a warm ocean about 200 million years ago, in the Jurassic period. You are near the equator and the sun is bright overhead. The ocean is warm and shallow. The ocean is growing wider as well, and the mid-ocean ridge where new crust is forming mixes iron and other nutrients into the water. Plankton is everywhere, supporting a rich ecosystem including ammonites – like squid but with hard coiled shells – hunted by fast swimming ichthyosaurs, creatures that look like modern dolphins but which are actually reptiles. The ammonites can’t eat all the plankton and it settles onto the sea bed where it forms a dark smelly mud made up of over 5 percent organic carbon.

As the millions of years pass, the mud gets buried and slowly heated. Plankton is different from peat. Chemically it is much more complicated and contains organic compounds like proteins and lipids that are long chains of hydrogen and carbon. As the mud turns into rock a substance called kerogen is formed as the organic compounds decompose. It may contain more Hydrogen than Carbon, but it’s composition is extremely varied.

Rocks containing kerogen can be found at the surface today – they are called oil shales. With a little effort they can be made to burn, but as they are they are not a useful fuel. To produce oil, kerogen needs to be cooked.

Cooking is using heat and sometimes pressure to turn organic matter into something useful to humans. Heating oil shale to around 60 to 120 degrees C produces petroleum (otherwise known as crude oil). This range of temperatures is known to oil geologists as the oil window. At higher temperatures it becomes over-cooked and natural gas is formed but not oil. These temperatures are not super high. You could cook kerogen in your kitchen, but I wouldn’t recommend it.

In the oil industry’s early history people extracted petroleum by directly heating oil shale. But they stopped when they realised that nature had already done it for them. Let’s go back to the oil shale that formed 200 million years ago. It’s now sitting deep under the Middle East, in Saudi Arabia or Iraq and it has been heated through the oil window. As a source of the oil, these layers are known as the source rocks. Oil is a light liquid, so it flowed up into the rocks above. Some of these contained spaces within them for the oil to sit in. The spaces are sometimes the gaps between grains in a sandstone, or maybe cracks in a limestone. These are known as the reservoir rocks. In many places the oil keeps rising up until it reaches the surface, where it might form a tar pit on the surface. Often the oil is stopped from rising up. This can be done by a seal or cap rock, a layer that it can’t flow through perhaps made from salt that evaporated from the ocean, or another layer of shale. The place where the oil accumulates, where it is caught beneath the seal, is known as a trap.

The way I’ve described oil being formed from the remains of past life is known as the biogenic origin theory. Against that, some scientists believe that oil or gas can form underground from carbon trapped deep within the earth. This abiogenic (“without life”) origin theory is not widely believed. We don’t well understand how carbon behaves deep in the earth and it’s not impossible that small amounts of petroleum form in this way, however there is a lot of evidence for the biogenic origin theory. The most convincing piece of evidence is that oil fields are never found in rocks that aren’t sedimentary ones. Igneous rocks, formed from molten rock, contain very little carbon and never any oil. Metamorphic rocks have been heated too much. Geologists have identified source rocks in metamorphic areas, but now they contain only graphite, a form of carbon that is no good as a fuel.   

Another line of evidence for a biogenic origin is that we can link oil in a trap to a particular source rock by studying chemicals called biomarkers. These are complex organic chemicals that have survived unchanged from the remains of life that formed the source rock. We can even use these to infer the age of the organisms that formed the oil. Some biomarkers are only found in oil from really old sediments, others in oil from recent sediments. As life evolved into new forms, so new biomarkers were created, to survive and be found as chemical fossils in oil.

Thick layers of sediments are found in places called basins – this is where most oil is found. Oil geologists investigating a new sedimentary basin will study it in a variety of ways. They will drill holes and look at the fragments of rock that come back to the surface as they go deeper. They’ll also perform seismic surveys by making small explosions and listening to the sounds that come back. Like bats using sonar, they can use this to understand how the layers look underground. They’ll also try and understand the geological history of the area. They are looking for suitable source rocks, with reservoir rocks above. The source rocks must also have been heated so they pass through the oil window. If any of these things are missing, oil will not be found. Finally oil traps must be found, which can be more difficult – there can be a lot of luck and drilling the holes to find them is very expensive.

Oil geologists are not looking so hard for new oil fields at the moment, as the price of oil is relatively low. The reason for this is the way in which new technologies, such as fracking have been used in the USA. Sometimes source rocks still contain large volumes of oil and gas that hasn’t migrated upwards. Fracking is using water and sand to fracture the source rocks and extract this oil and gas from a small area. This technique can be used in areas without ‘conventional’ oil and gas reserves, where it is pumped out of the reservoir rocks via a single oil well.

Fracking has transformed our understanding of the amount of oil that can usefully be extracted. Before fracking changed things, some experts believed that we had reached ‘peak oil’, a point where global supplies of oil had started to run out. This idea was based on studying individual oil fields, where production reached a peak and then afterwards rapidly declined. They believed that known oil fields were close to their peak and that new ones were not going to be found. Fracking has changed this. It turns rocks previously thought to unsuitable for extracting oil into rich sources of it. Estimating the global amount of oil remaining is extremely difficult – new technologies might continue to improve our ability to extract oil more efficiently.

Just because we can find and extract more oil doesn’t mean we should.  There are very important reasons to leave as much oil as possible within the ground. To explain why we need to understand the carbon cycle.

The Carbon atoms inside your body got there via food you ate that helped you grow. The atoms themselves were made in the heart of a star billions of years ago. Since then they haven’t changed at all, only moved around. They made their way to earth and since then they have moved from place to place countless times. The way in which carbon moves around the earth is known as the ‘carbon cycle’.

Part of the carbon cycle is purely biological. A plant pulls CO2 from the air and uses it to build organic compounds and grow. You eat the plant and digest it, turning it into energy storing chemicals in your body. Inside you body, you use Oxygen (taken from the air via your lungs and blood) to react with those chemicals, producing energy and CO2 that returns to the air. Some Carbon atoms inside you right now may once have been part of a famous person, or a dinosaur.

The geological part of the carbon cycle is also complicated. Carbon captured by plants or plankton can be buried underground and so does not return to the atmosphere for a long time. Limestone is a rock made of the remains of animals that build shells or structures out of calcium carbonate, CaCO3, which is another way of ‘fixing’ carbon and removing it from the atmosphere.

If the carbon is swallowed in a subduction zone, like under the Andes in South America, it may be buried hundreds of kilometres under the ground. There some of this Carbon is converted into precious diamonds. Later some of these diamonds return to the surface via odd volcanic eruptions. Places where oil seeps to the surface or coal or limestone is eroded and turned back into CO2 complete the geological cycle, putting the carbon back into the atmosphere. The Carbon may have been buried for hundreds of millions of years, but it returns eventually. No doubt some carbon atoms have followed that journey many times.

We know from studying ancient climates that levels of CO2 in the atmosphere have varied over time. We can also see that the climate was hotter when CO2 levels were higher. We can also see that the changes are very very slow. Natural erosion of limestone, or of coal or oil seeps increase CO2 levels but slowly, over millions of years.

Over the last few hundred years, humans have changed the carbon cycle in a really dramatic way. Through the burning of fossil fuels, huge amounts of Carbon that was buried for hundreds of millions of years have been returned to the atmosphere.

It’s hard to describe how quickly this change has happened, from the Earth’s point of view. Carbon that was produced by 100s of millions of years of sunlight has been released to the atmosphere over 100s of years. The earth has never seen anything like this before.

First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. Reproduced with permission.

The Holocene: from the Ice Age to the Age of Rice

Posted on by

Every day history is being made. This is true for geology as well. Out at sea layers of sand or mud are being laid down that will become the geological records of the future. The period of geological time that we are in right now is called the Holocene epoch, which is part of the Quaternary period, which is part of the Cenozoic Era. Let’s take a step back and tell the story of how we got to the Holocene.

The Cenozoic Era started 66 million years ago with the extinction of the dinosaurs, at a time when massive volcanic eruptions and a huge meteorite impact caused enormous changes to life on earth. With the dinosaurs gone (apart from birds, which are types of dinosaur – just look at their feet!) mammals were able to evolve into new species. The earth’s tectonic plates continued their slow dance. The Atlantic ocean continued to open, splitting Europe from North America (they used to be joined together). The Indian plate (which used to sit alongside Africa, Antarctica and Australia) moved quickly north and around 60 million years ago punched into the Asian plate. This started to form the huge mountains of the Himalaya, the high Tibetan Plateau and the the Tian Shan. Moving through the Cenozoic, animals and plants and the geography of the earth become more familiar. Grasses first become important around 40 million years ago. Also animals that eat grass, like horses and cattle. One thing that was different was the climate, which was much hotter, with higher levels of CO2 in the atmosphere and no ice caps.

Significant glaciers started to form in Antarctica around 34 million years ago. By the Quaternary period the earth is in an Ice Age, with large ice caps on Greenland and Antarctica and permanent glaciers in high mountains around the world. The Quaternary started 2.588 million years ago and it hasn’t ended yet. There have been ice ages before, during the Proterozoic – when the earth nearly froze completely – also in the Ordovician and the Carboniferous. These are times when the levels of CO2 in the atmosphere are relatively low. As ice caps lock up lots of water, sea levels tend to be low also. The amount of ice is relatively sensitive to changes of the earth’s spin causing changes in the amount of sunlight the earth receives. Tiny variations in the spin of the earth, caused by the irregular tug of other planets in the solar system are called Milankovitch cycles. There are different Milankovitch cycles that move at different speeds, so the pattern is complicated, like listening to many drummers who are moving at different speeds. The main effect is a pulse of between 40,000 to 100,000 years. We see this as dramatic changes in the volume of ice and of the climate of the earth. These are called glacial and interglacial periods and the earth has moved back and forth between them through the Quaternary.

We know what an interglacial period looks like, as we are in one right now. During past glacial periods, New York was covered by a layer of ice many kilometres thick, as was northern Europe. In China, the ground was permanently frozen as far south as Beijing and most areas were desert. Finely ground rock, from ice caps and glaciers further north or in the Himalayas was blown across this desert building up thick deposits of a fragile yellow soil called loess. An area called the Loess plateau covers the upper and middle parts of the Yellow River and makes excellent farmland. The soil erodes easily and the Yellow River’s name comes from the large volumes of eroded loess that it contains.

The Holocene epoch started 11,700 years ago and it’s the period of time since the end of the last glacial period. During it the climate has been relatively stable, with limited ice cover. But there have been many changes over that time. The first half of the Holocene saw areas that had been covered by ice slowly move to more modern conditions. Sediments in lakes contain ancient pollen and allow the plant species in the area to be identified. In many places there was a slow return of forests, with more and more species becoming established as thousands of years passed. Tropical rain forests, like the Amazon, were quite restricted during the glacial period and they also grew rapidly during the early Holocene. Some climates were very different. Parts of what is now the Sahara desert in Africa was a fertile place during the Holocene. Here we have evidence from ancient humans. Cave paintings show the hunting of giraffes and other large animals in places now barren desert.

Human beings are the most remarkable thing about the Holocene. The history of human evolution is complicated. Creatures clever enough to make stone tools have been around for 2.5 million years, but the oldest known fossils of modern humans (Homo Sapiens) are 300,000 years old. By 50,000 years ago humans had spread from Africa across much of the world and were using sophisticated stone tools and burying their dead. They were as clever as us, but had very little technology to help them.

Even with simple stone tools, humans made an impact. The ground in Siberia is frozen. Within it people find the corpses of extinct animals, like giant wooly elephants called mammoths. These animals used to be extremely common, but they became extinct during the early Holocene. Why? The changing climate might be responsible, but some believe that intense hunting by human beings is the main reason. In many sites across the world we have evidence of humans killing and eating large numbers of mammoths and other extinct animals. There is a debate about the most important cause of extinction, but it’s clear that even simple stone tools could be extremely effective.

Also human brains can be deadly. One effective method of hunting mammoth and other animals that live in herds was to scare them and drive them over cliffs, where they fell and died. During the Holocene humans took advantage of the improving climatic conditions to develop more and more new skills and technologies. The impact of increasing numbers of humans and their increasing ability to change the earth and the animals and plants upon is a theme of the later Holocene.

When populations of humans are small, we can live by hunting animals such as mammoth and gathering wild foods, like nuts or shellfish. Such foods are rare, so groups of humans would often move, following migrating animals, moving to different foods sources throughout the year or simply moving to new places. From the early Holocene we have evidence in the Middle east of the first villages or towns. For people to become settled, we need a reliable source of food that can be stored to provide food all year round. These first villages were based on harvesting grains from grasses – wild versions of modern day wheat or other cereals. Over time, selection of the best wild grains lead to the breeding of domesticated versions. These may require human intervention to thrive, but gave a good yield of food. A similar process occurred with animals. Wolves became dogs, who could help defend herds of domesticated sheep from attack by non-human predators. Hunter gatherers became farmers. Over time the same process affected fruit and other vegetables. Plants with small bitter fruits were slowly changed by selecting the best ones to end up with the large and juicy fruits we enjoy today. Very little of what we eat today is truly wild.

This process happened in different areas based around different foods. People in Central America domesticated maize from around 8000 years ago. In China and southeast Asia people started growing rice and eating it from around 7500 years ago.

Once people have worked out how to grow reliable sources of food, they can move across the land, spreading their seeds and animals as they go. We can trace this via records of pollen in lake sediments and other wet places. In places that were previously covered in trees, farmers would remove them, dramatically changing the landscape. The natural regeneration of forest, with seeds growing into trees is broken by domesticated animals like sheep eating the young trees. The wide open mountains and fields of the British countryside, for example are entirely man-made.

In a group of hunter-gatherers, groups of people tended to be small and most were involved in producing food. As farming villages became towns, a group of people emerged who did not need to directly work to produce food. These people – kings, priests, merchants, craftsmen and others had the time to produce new things, like writing, organised religions, metal objects and so many other things. In some places the nature of the farming required many people to work together. In the Mesopotamian basin (modern-day Syria and Iraq), where some of the earliest towns were formed, systems of irrigation were required to get water from narrow rivers into broad areas of farmland. Technology itself sometimes also encouraged the growth of large kingdoms. Bronze is a metal alloy produced from a mixture of copper and tin. In the eastern Mediterranean of Europe there is copper, but the tin had to come from far away, like Afghanistan or the British Isles. Only large kingdoms or empires had the resources to bring together such distant materials.

The period of time when people are using Bronze is called the Bronze Age and it was followed by the Iron Age and so we are moving into history rather than geology. Let’s take a step back and consider this enormous change in human activity from the point of view of the wider planet. Even at the start of the Holocene, humans may have been causing extinctions of animals. The rate of extinction has only grown since then, with no sign of stopping. Direct hunting is not the only cause. Farming landscapes are very different from ones untouched by man and while some animals have adapted (rats, for example), many have not.

Some human impacts on the planet that we think of as modern are surprisingly old, for example pollution. Ancient layers of ice in the Greenland ice cap are a record of atmospheric pollution. Each layer is a record of ancient snow, plus trapped bubbles of air. Studying this record shows traces of lead pollution from Roman mining in Spain from over 2000 years ago. Similar records from South America show the traces of local mining from 3500 years ago.

These ancient traces of human activity are dwarfed by the current impacts. During the Quaternary and most of the Holocene, CO2 levels slowly varied in step with Milankovitch cycles. Human burning of fossils fuels has dramatically changed this. Levels of CO2 are now higher than they have ever been before in the Holocene or even the Quaternary. Some geologists argue that the Holocene is over and that we are now in a new geological Epoch, that they call the Anthropocene.

First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. Reproduced with permission.

The geological history of the earth

Posted on by

To understand the past, we divide history into different pieces, some big, some small. Human history has been divided into Ages, (e.g. Stone Age, Bronze Age) then smaller periods like Dynasties, then by the reigns of single rulers. Geologists deal with much longer periods of time, but they divide the history of the Earth in a similar way. A trained archaeologist can find a piece of pottery and know that it was made during a particular period of time – the Ming Dynasty say. Geologists use fossils – the remains of ancient animals – in the same way. To find out how, let’s learn how about the people who first discovered how to do this.

Nineteenth Century Britain was a time and place where rapid development of industry and advances in science went hand in hand, each helping the other. William Smith worked as an engineer involved with canals and coal mines and so saw a lot of sections cut through rocks. He realised that he saw the same layers in different places, always in the same order and with the same types of fossil in each layer. He proposed that this was a universal scientific “law of faunal succession”. In any series of layers of sedimentary rock, as you look at each layer going up, fossils will appear in a specific reliable order. There are fossils in the coal-bearing rocks that are not seen in the younger limestones higher up, instead different ones are seen, sticking out of the walls of buildings in Oxford and Cambridge Universities. The rocks on top – the youngest – like the Clay under London contain a different set of fossils again. William Smith was the first person to create a geological map of Britain, that was published in 1815. He used his understanding of the fossils and his law of faunal succession to help him create the map. He produced ‘cross-sections’, pictures of a vertical slice down through the earth. These are of practical use. Dig down into rocks that you know sit above rocks that contain coal and you can make a mine. Dig down into older rocks and you are just wasting your time.

Modern scientists would use different words, but William Smith’s ideas are now known to be correct and useful in understanding geology across the world. Stratigraphy is the modern name for the study of layers of rock. Nicholas Steno, a European scientist working a century before William Smith first defined the laws of stratigraphy. These ideas are simple. Think about an old house that has been decorated many times. The walls are covered in many layers of paint, each on top of the other. If you think about it, it’s obvious that the layers closest to the wall were painted on first, and the ones above later. Think of a nail stuck in the wall. It will cut the layers of paint that were there when it was banged in. If it’s covered by different layers, then they were laid down after the nail was put in. Apply these ideas of layers of rock laid down on the surface of the earth and you get the laws of stratigraphy. Using fossils to understand these layers is known as biostratigraphy.

European scientists in the generation after William Smith studying rocks in other countries found the same fossils, but in different types of rock. What might be a limestone in one country could be a mudstone in another, but with the same types of fossils. Realising that these rocks were of the same age they started naming periods of time defined by the fossils. These are the geological periods that you may be familiar with. The oldest rocks that contain obvious fossils was named the Cambrian period after the name for Wales (part of Britain) in Latin (an ancient European language). The Ordovician and Silurian were named after the Latin names for ancient peoples from Wales. The Devonian was named after a part of Britain called Devon, the Carboniferous after the element Carbon as these rocks are rich in coal. The Permian is named after a Russian city, the Triassic in Europe contains a series of three different types of rock and ‘tri’ means three in Latin. The Jurassic was named after a mountain range in France, the Cretaceous after the latin for chalk, a rock common at this time.

The geological periods were being named when Charles Darwin was a young man, studying geology and other sciences. For example he studied with Adam Sedgwick who soon after named the Cambrian period. At the time, people were realising that the earth must be very old. They could see that layers of rock were kilometres thick. The rocks themselves were like like layers of sand seen in the modern sea or rivers. Knowing that layers in the modern world form slowly they realised that kilometres of rock would require millions of years to form.

A world that was millions of years old. Fossils that changed gradually over time. These are the ideas that were in Charles Darwin’s head as he sailed across the world studying modern plants and animals (and Geology) on a sailing ship called HMS Beagle. All these experiences led him to produce his theory of evolution by natural selection, the foundation of modern biology.

The theory of evolution also explains why biostratigraphy works. As animals and plants slowly change and evolve into new species, the fossils found in rock layers also change. Once an animal becomes extinct it is never seen again.

We now know that the way animals and plants change over time isn’t always a calm gradual process. Mass extinctions are events where many types of creature die out, due to meteorite impacts, or massive volcanic eruptions or other reasons. Many of these sit on the boundaries between different geological periods. The extinction of the dinosaurs happened at the Cretaceous-Cenozoic boundary. The biggest extinction ever – sometimes called “The Great Dying” – happened at the transition between the Permian and the Triassic. At this time 96% of marine species became extinct as massive volcanic eruptions poisoned the air and the seas.

Geological timescales – taken from Wikipedia

Often geologists talk about how many millions of years old something is. We were only able to measure the age of rocks in the Twentieth century, once we understood radioactivity in rocks better. Using fossils to divide time doesn’t require you to know exactly how old they are, just that this rock is older than that, or that these are the same age. Geological periods are the most familiar divisions of geological time, but there are others, some bigger, some smaller. In the nineteenth century, all rocks older than Cambrian period were lumped into the ‘PreCambrian’ and were thought to have no fossils at all. Now we are able to find out the ages of rocks without fossils, using radiometric dating. PreCambrian rocks make over 85% of the history of the earth, so geological periods called Eons are used to divide up this vast time. All of the Cambrian and later are known as the Phanerozoic eon, the word means ‘visible life’ in the Greek language. The eon older than this, from 2500 to 541 million years ago, is the Proterozoic, meaning ‘earlier life’. Even older rocks are from the Archean eon, meaning ‘beginning’. Rocks on earth older than 4 billion years old (they are very rare) come from the Hadean eon. The earth at this time was extremely hot, covered in molten rock and hit by frequent meteorite impacts, conditions seen as hellish. Hadean is named after Hades the Greek god of Hell.

Coming down a step, between the vast Eons and the more familiar Periods, we have Eras. The Proterozoic is divided into three, early, middle and late, or Palaeoproterozoic, Mesoproterozoic and Neoproterozoic. The Phanerozoic (Cambrian and later) is also divided into old, middle and new. These eras are the Palaeozoic, during which life first left the seas onto land, Mesozoic, when the dinosaurs roamed the earth and the Cenozoic when mammals became dominant.

The better known periods of the Phanerozoic are divided up still further into Epochs. Typically a period is divided into 2 or 3 epochs, often early, middle and late. Our next divisions are called Ages. Let’s get into some examples. The Cambrian period was named after Wales in Britain. Its youngest Epoch is the Furongian, meaning Lotus, another name for Hunan province in China. The Furongian Epoch is split into 3 Ages, the first two named Paibian (named after a village in Hunan Province) and Jiangshanian (named after a village in Zhejiang Province). The third Age of the Furongian Epoch is not yet named. All of these divisions of time are known as ‘chrons’. The term can be used to refer to any slice of time that can be well defined, even those shorter than geological Ages. Why are some Epochs and Ages named after villages in China? It’s because that when dividing finer and finer periods of time, it’s important to have a well-defined definition that can be used in rocks across the world.

Wales has lots of rocks of Cambrian age, but Hunan province in China has some of the best sequences of rocks from the Furongian Epoch. What geologists are looking for are continuous sequences of rocks rich in marine fossils that change rapidly over time. It’s not uncommon for piles of sedimentary rock to have periods of time when no sediment was deposited, when the record is broken. The Furongian Epoch, starts with the Paiban Age. It’s defined officially defined as the first appearance of a fossil trilobite species, called Glyptagnostus reticulatus (no, I don’t know how to pronounce it either). This animal was a little like a woodlouse that lived in the seas, widely across the planet. It’s found today in six different continents and so a perfect way to divide up time. The official reference point that defines the start of the Paiban is a sequence of rocks near the village of Paibi. This place, called a GSSP (for Global boundary Stratotype Section and Points) was chosen by a global group of geologists called the International Commission on Stratigraphy (ICS). Their mission is to define the boundaries between all geological Ages in terms of specific fossils and a place that best shows the boundary. More poetically (but not accurately) GSSPs may be called ‘golden spikes’, a place where humans have nailed down the flow of time to particular place and event. The work of the ICS is not complete, they have more ‘golden spikes’ to define. Maybe they’ll put one near where you live? Maybe they already have.

Not all golden spikes are defined by the appearance of disappearance of fossils. The boundary between the Cretaceous Period and Palaeogene Period is defined by a layer enriched in Iridium a rare element on earth but more common in space. The layer was formed by a massive meteorite impact (the crater is in the Mexican Gulf in North America). This is also when the dinosaurs became extinct which is probably no coincidence).

The basic principles of stratigraphy – layers above other layers are younger, for example – are universal. What if you had a planet with no fossils, that you’ve never visited but where you had a good set of photos sent by a robot, could you define geological Periods? Of course! We’ve done it for Mars after all. Martian geological periods, the Pre-Noachian, Noachian, Hesperian and Amazonian cover the same period of time as earth ones, but are entirely different. They are far less well defined too (no Epochs or Ages) but they answer the same human questions – How old is that? What’s its history? What stories can we tell from it?

If intelligent beings in the future were to look at the earth, they’d be able to use stratigraphy to understand the geological history of our times. Probably they’d use the same ideas and look for extinctions of animals, or unusual layers of rock to divide up time. What would the layers being laid down right now look like? They’d be interesting, I think.

First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. Reproduced with permission.